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The New Vital Sign Parameter


A 58-year-old man experiencing “chest pain and shortness of breath” calls EMS. By the time EMS arrives, the man is unresponsive with extremely labored breathing. The patient goes into cardiac arrest en route to the hospital and is ultimately pronounced dead in the emergency department (ED).

His brothers and sisters are notified and begin to make arrangements for his funeral. Family members travel to his city and stay in his home for the funeral events. When the family members fail to show up for an early viewing, the funeral director goes to the deceased patient’s residence and finds the family members dead in the home. It’s later determined that the man and his entire family died as a result of carbon monoxide (CO) poisoning caused by a malfunctioning furnace.

If the original EMS crew was equipped with a means of detecting dangerously high CO levels, these otherwise healthy adults wouldn't have perished.

This incident, and many more like it, point out the benefit of CO monitoring as the latest vital sign available to EMS. Although most states have already adopted the use of pulse CO-oximetry as a modality for both BLS and ALS crews, several are still debating whether to allow BLS providers to use this new level of monitoring. CO monitoring is useful for providers at both skill levels.

In appropriate patients, CO assessment meets the definition of a vital sign. When measured rapidly using a reliable monitor device, CO levels are used by emergency providers to guide triage, therapy, transport and the effect of care. This vital sign also guides the emergency provider to consider the possibility of CO poisoning in any other person who has been around the patient when the exposure was occurring. The use of a pulse CO-oximeter as the single triage monitor will provide the three elements of the emergency vital signs—pulse, oxygen saturation and carboxyhemoglobin (COHb) levels—needed to manage many rescuers during incident rehabilitation.

Old & New Methods
In the past, testing for CO poisoning traditionally consisted of a blood test conducted in the ED. These tests were first performed with arterial blood but more recently with venous blood. CO that enters the body combines to form COHb, and the blood test measures the percentage of hemoglobin that is occupied by CO, instead of oxygen. If available at the hospital, the blood test may take 30 minutes to perform, potentially delaying treatment for the patient.

Another method of measuring CO is a breath sampling, performed with an inline detector. Using a method similar to end-tidal carbon dioxide (or ethanol) measurement, exhaled air is analyzed and the partial pressure of CO calculated. This correlates with the amount of CO in the alveoli and, therefore, the bloodstream.

New transcutaneous monitors that measure CO are now on the market and in use, assisting in timely patient evaluation. Monitor devices using pulse CO-oximeter technology utilize multiple wavelengths of light to provide a noninvasive measurement of carboxyhemoglobin in seconds, in addition to oxygen saturation and heart rate. In the prehospital market, monitors have been introduced with appropriate protocols for screening related to CO exposures.

These noninvasive monitors are being used more frequently in the EMS community and improve care when used correctly, and matched with appropriate care protocols.

How It Works
The pulse CO-oximeter produces a concurrent oxygen saturation and COHb level to assist in the evaluation of a patient’s ability to carry oxygen on the hemoglobin molecules to organs and tissues. Elevated levels of carboxyhemoglobin have been associated with short- and long-term problems, including death.

Inline exhaled CO measurement has been available for some time, but has limitations for use in the emergency setting. The technology of pulse CO-oximetry is based on decades of work with pulse oximetry and the use of additional wavelengths of light to identify and quantify additional blood properties.

Combined with a sensitive optical absorber, digital filters and signal extraction technology, a mathematical algorithm calculates the levels of various hemoglobins in the pulsatile blood, and reports four types of hemoglobin:

  • Oxyhemoglobin;
  • Deoxyhemoglobin;
  • Carboxyhemoglobin, and
  • Methemoglobin

The values reported on the screen include oxygen saturation (SpO2), pulse rate, COHb percentage (SpCO), and methemoglobin percentage (SpMet). This same technology is also now available as an option in selected cardiac monitors.

Extensive research has correlated COHb levels reported on the monitor to arterial samples drawn concurrently.(1–5) In addition, clinical research from multiple sites across the U.S. and Europe has demonstrated that carboxyhemoglobin levels as measured by pulse CO-oximetry closely correlate to those measured using hospital-based technologies.(4–9)

Like pulse oximetry, the device has been adopted in emergency care for prehospital and hospital use, and the elements of the monitor have been revised and updated as the manufacturer receives feedback on the use in the field.

Using the Device
Using a pulse CO-oximetry device requires some precision; like other noninvasive devices, it can produce erratic results when used incorrectly. Some tips and cautions are below.

  • There can't be too much external light interference.
  • The finger must be positioned to the end of the digit stop in the sensor.
  • Adult sensors shouldn’t be used on children or those with small fingers.
  • The subject should remain still during the monitoring process.
  • With individual variations in CO clearance, it isn’t possible to compare SpCO readings with invasive COHb readings unless the invasive blood draw is simultaneous with the SpCO measurement.

As with other noninvasive monitor devices, it’s critical to use the results of a single reading in the proper context and to verify an unusual value with additional measurements before altering patient management. Pulse oximeters have been found inaccurate through certain fingernail polishes, artificial fingernails and malformed fingertips.

If a high value appears in patient screening, repeat the measurement on another finger or two to confirm. Remove the patient from the possible source of poisoning. Treat with supplemental oxygen and transport to an appropriate hospital. The value should be verified with a blood draw and determination through a blood-gas analyzer.

For guidelines on appropriate treatment for specific levels of CO, see "Sneak Attack," p. 4.

The product specifications for the CO-oximeter produced by Masimo Corporation are about 3% at one standard deviation, meaning the limits of agreement in about 95% of all measurements are about 6%. The limits of agreement in most published studies comparing blood results with finger probe values have been within 6%.(1-5)

A few studies have shown wider variation in the results in clinical use, but the methods sections of these studies don't establish whether the device was used appropriately, or whether the invasive blood sampling was done at the same time as the pulse CO-oximeter reading.

In the studies conducted to submit the device for FDA clearance, the CO-oximeter has exhibited accurate readings for COHb values between 0 and 40%. Therefore, it’s reasonable to conclude that when applied properly, SpCO measurements have acceptable accuracy and are clinically helpful.

In emergency care, patient-oriented outcomes are the endpoint of the use of our vital signs and monitors. A noninvasive SpCO measurement allows the emergency provider to determine what contribution is likely from carbon monoxide poisoning to the clinical condition being evaluated.

Smokers have been found to have CO levels of up to 8%, but otherwise the non-smoking population has CO levels less than 5%. Treatment is generally indicated when the SpCO exceeds 10 to 12%. Severe CO poisoning is likely for patients who have altered mental status or a history of a loss of consciousness, neurologic deficits, cardiac symptoms, and patients with CO levels of 20–25%. Because of the risk to the fetus, patients who are pregnant and have a COHb level greater than 15% should be considered dangerously poisoned.

A value within 5% allows the providers to establish a patient-oriented outcome, when combined with the clinical symptoms and the setting of the patient. A pulse CO-oximeter value in the single digits effectively excludes carbon monoxide poisoning. A value above 10% raises the issue of exposure, and the need for oxygen treatment.

A value above 20% establishes a high-risk poisoning, and the need for oxygen treatment and consideration of removal to a center with hyperbaric oxygen capability. A value above 30% is usually in a patient who has frank and obvious symptoms of CO poisoning.

The emergency provider will note the common symptoms, including history of unconsciousness, lightheadedness, ongoing altered level of consciousness, shortness of breath, chest pain, headache, nausea, seizures, cardiac dysrhythmias, and cardiac ischemia. Incidents where CO poisoning are expected include events where there has been enclosed use of hydrocarbon-powered machinery or power generators, malfunctioning natural gas or propane-powered home appliances, poorly functioning automobiles, and smoke inhalation.

Because the signs and symptoms of CO poisoning are so vague and nonspecific, CO exposure and poisoning is easy to miss. Pulse CO-oximetry provides unmatched screening accuracy in the field and can help eliminate previously misdiagnosed “flu-like illness.”

Missed CO poisonings are a particular area of legal liability for fire and emergency personnel. Failing to detect and diagnose CO poisoning can result in individuals being allowed to return to the contaminated environment, with devastating outcomes. As we become more adept in the use of CO-oximetry, we will improve the accuracy of the results and our ability to screen at-risk patients to prevent unnecessary suffering and death.

Adding to the Basic Toolkit
Many EMS, fire, hazmat, rescue and tactical response systems now use the pulse CO-oximeter as a tool for the assessment of all patients with symptoms of general illness, headaches, flu-like illnesses or symptoms of suspicious origin. Patient management can then be directed toward appropriate oxygen or hyperbaric therapy. When combined with the presenting symptoms and characteristics of the patients, the pulse CO-oximeter can also serve as a protocol guide to the utilization of regional hyperbaric oxygen chambers.

The benefits to EMS of pulse CO-oximetry are expanding. It can be effectively utilized in the management of single patients, performance of triage at mass casualty incidents (MCIs), rehabilitation of emergency personnel and public health screening.

Monitors are becoming ubiquitous, but some EMS personnel are not permitted to use pulse CO-oximeters or an ambient air hand-held CO monitor. In fact, members of the fire service are permitted to use CO-oximetry in some regions while EMS crews are not. This can, and must, be corrected.

These scope-of-practice issues—which involve differing protocols for EMT-Bs, EMT-Is and paramedics—are highlighted by technological advances. Just as there were hurdles for EMTs when AEDs were introduced to the public and pulse oximetry was added to the ALS armament, whether EMTs should be allowed to place CO-oximetry devices is also debated in some states.

The issues can be grouped in two areas: the “scope of practice” defined by each state and territory’s law or rule, and the appropriate medical directives for use in online or offline medical control.

To maximize the patient care capabilities in the future, EMTs at each level must have the ability to add more diagnostic capabilities, especially non-invasive items, and then use existing and future technologies for treatment. This would meet aspects of the National Blueprint for EMS, enhance the National Scope of Practice and provide appropriate advances to feed the education standards being created.

New—and more rapid—vital sign detection will prove to be an important adjunct for prehospital and hospital, clinic and public health providers. Transcutaneous pulse CO-oximeters have already proven to be an asset to the assessment and evaluation of patients, and this simple-to-use tool will continue to improve care by ALS and BLS crews if applied with appropriate protocols.

Pulse CO-oximetry allows us to revise our thinking about exposures, related diseases and occupational risks. Transcutaneous analysis for key substances and toxins will continue to expand, and we should expect to have more tools of this nature introduced in the future for prehospital care.

States that currently don’t allow EMTs to utilize CO-oximetry must independently address the scope-of-practice issue. State EMS offices should look to their colleagues, as well as regulation changes made in neighboring states, when evaluating the capabilities offered by this new vital sign parameter. CO-oximetry is simply too valuable a tool not to be in the BLS tool bag.

Disclosure: Dr. Augustine served as a consultant to Masimo Corporation in the development of CO monitoring technology. He reports no ownership interest or ongoing relationship with the company.

1. Barker SJ, Curry J, Redford D et al. Measurement of carboxyhemoglobin and methemoglobin by pulse oximetry. Anesthesiology. 2006;105:892–897.
2. Coulange M, Barthelemy A, Hug F et al. Reliability of new pulse CO-oximeter in victims of carbon monoxide poisoning. Undersea Hyperb Med. 2008;35(2):107–111.
3. Mottram C, Hanson L, Scanlon P et al. Comparison of the Masimo Rad-57 pulse oximeter with SpCO technology against a laboratory CO-oximeter using arterial blood. Respir Care. 2005;50:1471.
4. Piatkowski A, Ulrich D, Grieb G, et al. A new tool for the early diagnosis of carbon monoxide intoxication. Inhal Toxicol. 2009;21:1144–1147.
5. Suner S, McMurdy J. Masimo Rad-57 pulse CO-oximeter for noninvasive carboxyhemoglobin measurement. Expert Review of Medical Devices. 2009;6:125–130.
6. Suner S, Partridge R, Sucov A, et al. Noninvasive pulse CO-oximetry screening in the emergency department identifies occult carbon monoxide toxicity. J Emerg Med 2008;34(4):441–450.
7. Chee KJ, Nilson D, Partridge R, et al. Finding needles in a haystack: A case series of carbon monoxide poisoning using a new technology in the emergency department. Clin Toxicol (Phila). 2008;46:461–469.
8. Hampson NB, Scott KL, Zmaeff JL. Carboxyhemoglobin measurement by hospitals: Implications for the diagnosis of carbon monoxide poisoning. J Emerg Med. 2006;31:13–6.
9. Cone DC, MacMillan DS, Van Gelder C, et al. Noninvasive fireground assessment of carboxyhemoglobin levels in firefighters. Prehosp Emerg Care. 2005;9:8–13.

This article originally appeared in the October 2010 JEMS supplement “The Silent Killer” as “The New Vital Sign Parameter: CO-oximetry should be in the BLS toolkit.”


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